<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<h1>0</h1>
<h2>0</h2>
<script>
  let count = 0;
  //开启定时器 每隔1秒调用一次
  //方法作为参数传递时不写()
  setInterval(f,1000);
  function f(){
    count++;
    let h = document.querySelector("h1");
    h.innerText = count;
  }
  let num = 0;
  //匿名方法开启定时器
  let timer = setInterval(function (){
    num++;
    let h = document.querySelector("h2");
    h.innerText = num;
    if (num ==50){
      //停止定时器
      clearInterval(timer);
    }
  },100)

  //开启只执行一次的定时器
  setTimeout(function (){
    alert("时间到!")
  },3000)
</script>
</body>
</html>